The present invention relates to a production and heat storage system for low-temperature chilled water, which enables fresh water or a heat carrier (heating medium) to be cooled down to around the freezing temperature, using a combination of two heat source units making use of a low-pressure refrigerant with a heat storage tank.
As known heretofore in the art, heat sources for air-conditioning purposes has been produced by a system wherein, as illustrated in FIG. 5, cold water is supplied to the low-temperature (condenser) side of a refrigerating machine and the chilled water obtained on the high-temperature (evaporator) side thereof is pumped up to a heat storage tank for recycling. This system is designed to store chilled water in the nighttime making use of inexpensive "midnight power" and utilize the heat in the daytime, and is thus efficient enough.
With the above-mentioned conventional system for producing chilled water, however, the lower temperature limit of chilled water for heat storage is at most ca. 5.degree. C.; chilled water of around 0.degree. C. cannot be produced due to a water-freezing problem. When water is cooled down to around 0.degree. C., the temperature of refrigerant used on the evaporator side of the refrigerating machine must be lower than that of the water to be chilled. Cooling the refrigerant down to a temperature of around 0.degree. C. may possibly be achievable by increasing the heating surface area of a heat exchanger to infinity. Due to a water-freezing problem, however, a currently available temperature difference between water and refrigerant is actually on the order of 3.degree. C.; in other words, the temperature of refrigerant is at least 1.degree. C. while the exit temperature of chilled water is ca. 4.degree. C. Furthermore, it is impossible to keep all the flow rates in heat exchanger tubes constant due to some limitation placed on the control mechanism of the refrigerating machine. Nor can the temperature of refrigerant be always kept constant. For these reasons and because of a temperature change of about .+-.0.degree. C. occurring actually, chilled water of around 0.degree. C. cannot be produced as yet.
In addition to cooling systems operated on condition that recycling fresh water may be frozen, there are some systems for producing chilled fresh water of 1 to 2.degree. C. with refrigerating machines and heat exchangers using a high-pressure refrigerant.
With these systems designed such that the recycling water to be chilled flows along the shell of the heat exchanger, however, it is impossible to obtain chilled water of 1.degree. C. or lower. Nor can a temperature difference as large as 100.degree. C. or more be removed by one-cycle cooling due to their single-stage cooling design. It is thus unfeasible to achieve heat storage at a temperature as low as 1.degree. C. or lower with enhanced efficiency, making use of a perfect mixing type of multi-tank unit often used as a heat storage water tank for cooling-and-heating purposes.